Ford has a way to protect knees in a crash with an inflatable air bag that will debut on the 2015 Mustang later this year.

The new air bag is smaller and lighter, which makes for a roomier interior. The key is an inflatable plastic bladder integrated into the glove box door instead of the usual knee air bag mounted under the instrument panel.

The injection-moulded plastic bladder replaces a traditional fabric air bag and is sandwiched between the inner and outer panels of the glove box door. When the car's sensors detect a crash is imminent, an inflator fills the bladder, extending the entire glove box outer door panel toward the front passenger’s legs, providing a protective cushion.

The location is ideal. The glove box is closer to the knees, so the bag does not have to travel as far, and the glove box door distributes the impact load over a larger area.

"With this innovative air bag design, we have developed technology that provides protection for vehicle occupants while also reducing weight and enabling greater design flexibility," said Sean West, Ford’s restraints manager.

The smaller packaging — it is 75 per cent smaller and 65 per cent lighter — allows designers to think differently about where they can place air bags in cars in the future.

There are no visible seams or air bag cover for a more attractive interior. Without having to worry about the logistics of the air bag in the dashboard, it can be pushed closer to the windshield for a roomier feel to the cabin.

It is part of a larger safety system with more sensors, the addition of seatbelt pretensioners that tighten the belt before a crash, and a doubling of the total number of air bags to eight in the pony car.

Ford has 15 patents on the system so far, and applications for more have been filed.